This invention relates generally to a cooking magnetron power supply system and, more particularly, to such a system wherein magnetron filament power is stabilized.
Most microwave ovens presently on the market use a 50 or 60 Hz LC power supply system along the lines described in U.S. Pat. No. 3,396,342 Feinberg issued on Aug. 6, 1968. This type of power supply, which is used in microwave cooking appliances from low power sub-compacts to combination electric range/microwave units, has existed for over twenty years.
Among the advantages of the Feinberg power supply system are the simplicity of using only four components and good control of the power factor. Disadvantages include the bulk (weight and size) need for controlling the power by the duty cycle only, noncontinuous filament power at power levels other than 100%, high in-rush current and lamination noise. The bulk disadvantage of the Feinberg system results from the requirement for a 50 or 60 Hz transformer rating of about 1.2 KVA. Iron and copper weight of such a transformer typically weighs about 700 grams and occupies a volume of 1710 cubic centimeters. Additionally, a physically large capacitor is required as a necessary component when using such a transformer in order to provide constant current regulation of magnetron power against variations in line voltage.
A push-pull system has been used or proposed in connection with powering a cooking magnetron. Although the push-pull system avoids some of the disadvantages of the Feinberg power supply arrangement, such a push-pull system has included disadvantages such as high cost, complex logic, high voltage Darlington connected power transistors, reactive (i.e., power dissipative) snubber networks, inherent unbalance in volt second characteristics for each half cycle of operation being caused by uncontrolled turned-off characteristics of switching transistors, poor input power factor (for example, 0.6), high EMI generation, poor conversion, and higher cost magnetics. The higher cost magnetics corresponds to a design having a variable leakage transformer as a means of power control.
The powering of the magnetron filament has presented some design problems in magnetron power systems. In particular, some magnetron power systems may result in undesirable power changes in the filament of the magnetron. More specifically, designs which have an arrangement for adjusting the output power of the magnetron itself (i.e., the microwave output due to power applied to the anode and cathode of the magnetron) may inadvertently change the power applied to the filament of the magnetron. Depending upon the technique used for controlling the magnetron power, the power in the filament of the magnetron may fluctuate over an undesirably wide range.
Accordingly, it is a principle object of the invention to provide a microwave energy generating system having a power supply wherein the magnetron filament is stabilized against fluctuations in its power applied to the magnetron.
A more specific object of the present invention is to provide a microwave energy generating system wherein the filament power is stabilized against changes which would otherwise occur as a result of an adjustment of the microwave output of the magnetron.